1. Field of the Invention
the present invention relates to piezoelectric transducers taking the form of an annulus and which are fitted with means making it possible to prestress this annulus so as to apply a stress of specified value thereto. It also relates to the processes which make it possible to implement these means for applying the said prestress to the annulus.
2. Discussion of the Background
Use is frequently made in underwater acoustics of piezoelectric transducers which make it possible to obtain acoustic waves, more particularly low-frequency acoustic waves, on the basis of an electrical excitation signal. A particular form of such a transducer, more especially suited to the emission of low-frequency waves, is that of a torus of rectangular cross-section, formed by a set of ceramic segments polarized head-to-tail and assembled by adhesive bonding with the interposition of an electrode between each segment. The segments thus excited contract and expand in tempo with the electrical signals which are applied by the electrodes, and this tangential motion of the segments translates into a radial extension and contraction of the annulus. This motion therefore gives rise to the production of acoustic waves which are emitted with radial symmetry about the axis of the annulus into the medium, generally the sea, in which the transducer is immersed.
To obtain sizeable acoustic power, the annuli are subjected to piezoelectric stresses of high amplitude and this effect is all the more marked the lower the frequency of the acoustic waves to be emitted. Under the effect of these stresses, the annulus would tend to break up, initially at the interfaces between the various segments and subsequently by straightforward rupture of the piezoelectric ceramics above a certain emission level. To alleviate this drawback it is expedient to prestress the annulus by compressing it with the aid of means which apply radial forces thereto, directed towards the centre and distributed uniformly over the outside surface of the annulus. These radial stresses induce tangential stresses which tend to hold the segments together securely and oppose the development within the ceramics of tensile stresses to which this type of material is known to be particularly brittle.
Various kinds of devices have been envisaged to obtain such stresses. These methods generally consist in winding a strap of an appropriate material around the annulus while pulling very hard on the ends of this strap so as to obtain a suitable hooping. Examples of these methods will be found for example in French Patents No. 2 346 862 and 2 463 979.
The methods thus used nevertheless have various drawbacks.
In particular the final value of the prestress thus obtained fluctuates within wide limits in an uncontrollable manner. These conditions, and since the system is neither dismantleable nor adjustable, lead to the scrapping of the annulus during construction while it is at a very advanced stage in its manufacture, thus giving rise to a considerable loss.
Moreover, given the various means making it possible to pull on the strap, as well as the friction of the latter on the surface of the segments, the stresses which are thus generated are not distributed uniformly and they are generally concentrated at a particular point corresponding to the stack of ribs. Such unevenness is a source of considerable hindrance, given the radial isotropy which it is sought to obtain for the acoustic radiation.
Furthermore these drawbacks are all the more considerable the larger the diameter of the annulus. Now, the diameter of the annulus is directly related to the desired frequency of emission. The lower the desired frequency the larger the annulus must be, and since in this case the larger the emission power desired the greater is the need for the prestress, and hence the more important become the drawbacks mentioned above.